1. Field of the Invention
The present invention relates to a focusing control apparatus used in a lens system in which a soft focus function for controlling a degree of soft focus as desired is incorporated in an optical system of a lens having a focus control function.
2. Related Background Art
The soft focus lens imparts a soft focus effect to a resulting photograph. The soft focus is attained with a desired degree by moving a portion of an optical system on an optical axis to change a spherical aberration.
A conventional focusing detection apparatus includes (1) a continuous mode in which focusing detection and driving of the lens into an in-focus position are always carried out during semi-depression of a shutter release button and the shutter is released irrespective of the in-focus state when the shutter release button is fully depressed, and (2) a single mode in which the focusing detection is carried out during the semi-depression and the release button is locked until the lens is driven to the in-focus position, and once the lens is in the in-focus position, the focusing detection and the lens drive are inhibited unless the release button is again semi-depressed, and the shutter is released when the release button is fully depressed in the in-focus state. Those modes may be selected as a user wishes.
When a lens system for the soft focusing is moved toward + or - on the optical axis, the focusing control information generated by the focusing control apparatus includes an error due to an affect of the spherical aberration of the soft focus lens, and accurate focusing detection and focusing control are not attained. Such an error occurs in both of the single mode and the continuous mode. This phenomenon is explained with reference to FIGS. 7 and 8.
FIG. 7 shows a change of a spherical aberration in the focusing detection by a focusing detection optical system for light beams corresponding to F-numbers F1 and F0 with zero drive amount of the soft focus lens. FIG. 8 shows a change similar to that shown in FIG. 7 when the soft focus lens is driven by a certain amount.
As seen from FIG. 7, where the soft focusing effect is not imparted, the spherical aberration is small and an optimum image plane does not significantly change by the F-number and the effect on the focusing control precision is small. However, where the soft focus effect is imparted as shown in FIG. 8, the spherical aberration is large and the optimum image plane significantly changes with the F-number and the effect on to the focusing control precision is large. If this is neglected, it is not possible to focus the lens to a principal object.
Where the soft focus function is used in the single mode and if the lens for the soft focus is driven toward + or - while the focusing lens is in the in-focus position, the defocusing due to the soft focus effect and the shift of the optimum image plane due to the change of the spherical aberration may be observed. In order to confirm the soft focus effect, the semi-depression of the release button is once discontinued and then the release button is again semi-depressed to drive the lens into the in-focus position in order to watch the degree of the soft focus. If the degree of the soft defocusing is excessive, the above operation must be repeated. Accordingly, the operability is poor.
Further, in the single mode, if the position of the lens for the soft focus is changed after the in-focus state is attained and the release button is depressed, a photograph with a shifted optimum image plane is produced.
It may be possible to select the single mode normally and switch the mode to the continuous mode only when the soft focus lens is used. However, this would require the manipulation of the soft focus lens as well as the mode switching, resulting in poor operability.